Suggestions on HP21 restoration

[Marcovecchio]

Blackjetrock, many many thanks for all the suggestions and videos. I have a lot to learn from you! You are already a master at the kind of project I'll attempt.

I think I already have a STM32 prototype board somewhere here, and it's small enough to fit the HP21 housing. I'll try it first, but I'll also consider the other options you mentioned, thanks!